Find the correct type of replacement for the defective limit switch. Turn off the furnace and remove the old limit switch by separating the connecting wires and unfastening the holding screws. Install the new switch, place the doors back on the furnace and turn the power back on.
First, check to make sure all the information for the new limit switch matches to the old switch. This includes model number, serial number, manufacturer and voltage ratings.
Next, turn off the furnace using either the breaker or the furnace service switch. Use both to be safe. Wait until it has completely powered off before proceeding with the replacement. Remove the doors to the furnace and locate the old limit switch. Remove it by taking out the holding screws and separating it from the attached wires.
Once the old switch is removed, attach the new limit switch to the wires. When the wires are held in place, put the limit switch in its designated location. Hold the new switch in place and fasten the screws into their designated holes. Close the doors to the furnace securely. Turn the power to the furnace back on and let it run for a minute as a test.